1. Field of the Invention
The present invention relates to a bandwidth allocating control apparatus, and more particularly to a bandwidth allocating control apparatus included in an optical line terminal (OLT) which is arranged in a central office of a communication carrier and connected to a plurality of optical network units (ONUs) located on subscriber premises by a passive optical network (PON) to form an optical access system.
2. Description of the Background Art
A PON system using a time division multiplex (TDM) technology is widely utilized as a system for inexpensively constituting an optical access network. As one of the TDM-PON system, the GE-PON (Gigabit Ethernet (trademark)-PON) system has been widely popularized in Japan.
In the GE-PON system, bandwidths for the up-stream direction are allocated to the respective ONUs connected to a telecommunications network. One of the methods for allocating bandwidths is dynamic bandwidth allocation (DBA). The DBA attains an efficient bandwidth allocation by an OLT grasping the amounts of traffic raised in the respective ONUs, or the amounts of transmissions requested by the respective ONUs. In this case, in order to report the amount of traffic from the ONUs to the OLT and bandwidths to be allocated from the OLT to the ONUs, REPORT and GATE frames, respectively, are used which are regulated by the IEEE (Institute of Electrical and Electronics Engineers) 802.3av and IEEE 802.3ah STD.
More specifically, ONUs transmit REPORT frames to the OLT in order to notify the OLT of the amounts of traffic raised in the ONUs. The OLT receives the REPORT frames from the ONUs in time slots respectively allotted in advance to the ONUs. The OLT proceeds to computation through the DBA to thereby assign bandwidths to the respective ONUs. The DBA is performed in periodic time intervals called grant periods. The bandwidths thus assigned will be used in grant periods following thereto for receiving information on the amounts of traffic from the ONUs. The OLT transmits GATE frames to the ONUs to notify the latter of the bandwidths thus assigned. Each of the ONUs will know from a GATE frame when to transmit a REPORT frame and up-stream traffic, or data, in the following grant period, and proceed to the transmission.
The GE-PON system is desired to expand its service areas. However, the extension of a distance from the OLT to an ONU may cause a longer time to be taken from transmission to arrival of GATE and REPORT frames, thus requiring a grant period to be lengthened to the extent of problematically delaying data to arrive. Note that this problem is raised not only in the GE-PON but also, for example, a 10GE-PON attaining a bit rate of 10 Gbps.
In order to expand the service area of a GE-PON system, U.S. Patent Application Publication No. US 2007/0122151 A1 to Watanabe proposes a bandwidth allocating control apparatus having its bandwidth divided into bandwidths for short and long distances, which will be allocated to the respective ONUs according to the distances from the OLT to the ONUs. That solution is also disclosed by Daisuke Murayama, et al., “Dynamic bandwidth allocation to extend the applicability domain of EPON” The Institute of Electronics, Information and Communication Engineers, Technical Report, CS2009-67 (January 2010), pp. 1-6.
However, in the bandwidth allocating control apparatus disclosed in Watanabe, and Murayama, et al., an allocatable bandwidth is divided into bandwidths for short and long distances, which will, respectively, be allocated to a short- and a long-distance ONU, i.e. optical communication units. Therefore, for example, when having allocated all the short-distance ONUs to appropriate bandwidths with an unallocated, or residual, bandwidth or bandwidths remaining, the remaining bandwidths would not efficiently be utilized. Thus, the existence of such residual bandwidths may reduce the bandwidth usability, which may be problematic.